Guys were running the 1/18th Tekin speeder in 1/12th Stock brushed last year with no problems. I'm sure it will happen for brushless too.
While I do know that some 1/12 guys were running the Tekin 1/18 BL speedo... with the release of the RS, many of them have already switched. The main advantage of the 1/18th speedo was is small size - however, now that the RS is actually here and it's size is probably one of the smallest footprints of all BL speed controls, the added performance gains and added flexibility of being able to run lower wind motors makes the investment into a RS way more sense.
Oh.. and awesome that the race is running a Rubber Tire class (just to bring it back on topic).